This invention relates to an idling rpm feedback control method for internal combustion engines, and more particularly to a method of this kind which is adapted to perform the rpm control in different manners dependent on the state of engagement of the power transmission means of the engine, when the engine is decelerating with the throttle valve fully closed and the engine rpm has fallen into the feedback control range, to thereby improve the fuel consumption and enhance the driveability of the engine.
In an internal combustion engine, the engine can easily stall due to a drop in the engine speed when the engine is operated in an idling condition at a low temperature of the engine cooling water or when the engine is heavily loaded with demands by the head lamps, air conditioner, etc., in a vehicle equipped with the engine. To eliminate such a disadvantage, an idling rpm feedback control method has been proposed (e.g. by Japanese patent provisional publication (Kokai) No. 55-98628), which comprises setting desired idling rpm in dependence upon load on the engine, detecting the difference between actual engine rpm and the desired idling rpm, and supplying a quantity of supplementary air to the engine in a manner responsive to a detected difference, to thereby control the engine rpm to the desired idling rpm.
According to this proposed method, if the idling rpm feedback control is carried out immediately when the engine is decelerated toward the desired idling rpm region and before the engine speed drops to the desired idling rpm region, the resulting controlled quantity of supplementary air is much smaller than a required quantity to be supplied to the engine, because on such occasions the supplementary air quantity is controlled to such a small value as to immediately bring the engine rpm to the desired idling rpm. If on such occasions the clutch of the engine is disengaged, there occurs a sudden drop in the engine speed, which can cause the engine to stall.
To avoid this disadvantage, an idling rpm feedback control method has been proposed, e.g. by Japanese patent provisional publication (KOKAI) No. 55-98629. According to this proposed method, in transition from a decelerating condition to an idling rpm feedback controlling condition, the quantity of supplementary air is controlled in decelerating mode, wherein supplementary air is previously supplied to the engine in a quantity gradually increasing with a drop in the engine speed before the idling rpm feedback control is started, to thereby ensure the smooth transition to the idling operation.
However, even if the above control manner is followed, sometimes the driver will maintain the clutch in an engaged state until the engine is about to stall, when the engine is decelerating with the throttle valve fully closed, and the engine speed drops below the engine rpm at which the idling rpm feedback control is initiated.
Even then, as the aforesaid feedback control manner is applied, the quantity of supplementary air supplied to the engine increases with a decrease in the engine rpm so as to bring the engine rpm closer to the desired idling rpm. As the quantity of supplementary air supplied is increased in this manner, if the clutch is disengaged, the engine load can suddenly drop, resulting in a sudden increase in the engine speed, which in turn can cause not only discomfort to the driver, but also can badly affect the fuel consumption and exhaust gas emission characteristics of the engine.